1. Field of the Invention
This invention relates to passivation of contaminant metals on cracking catalysts. More specifically this invention relates to an improved method for passivation of contaminent metals on zeolite cracking catalysts.
2. Prior Art
In a fluid catalytic cracking process, hydrocarbon feed material is cracked at elevated temperature in a reactor containing a fluidized catalyst therein. Several such cracking catalysts are available and comprise acid-activated clay and zeolitic catalysts, although the predominant type is the zeolitic catalyst. Catalytic cracking may also be carried out in a so-called "moving bed" unit wherein catalyst pellets move downward through rising, hot gaseous hydrocarbons. As the cracking process continues the activity of the catalyst gradually deteriorates. Fluid catalysts are typically removed, regenerated in a regenerator to burn off coke and provide heat for subsequent cracking reactions and returned to the reactor. In the regeneration step carbonaceous materials deposited on the catalyst during cracking are burned off with air. Typically the process may be run continuously with catalyst being drawn off continuously from the reactor, regenerated and returned to the reactor along with fresh catalyst added to make up for stack losses or to boost equilibrium activity.
The catalyst cannot be regenerated to the original activity level indefinitely, even under the best of circumstances, i.e. when accretions of coke are the only cause for reduced activity. When activity has deteriorated sufficiently zeolitic catalysts must be discarded.
Loss of activity or selectivity of the catalyst may also occur if certain metal contaminants arising principally from the hydrocarbon feedstock, such as nickel, vanadium, iron, copper and other heavy metals, deposit onto the catalyst. These metal contaminants are not removed by standard regeneration (burning) and contribute markedly to undesirably high levels of hydrogen, dry gas and coke and reduce significantly the amount of gasoline that can be made. Contaminant levels are particularly high in certain feedstocks, especially the more abundant heavier crudes. As oil supplies dwindle, successful economic refining of these heavier crudes becomes more urgent. In addition to reduced amounts of gasoline, these contaminant metals contribute to much shorter life cycles for the catalyst and an unbearably high load on the vapor recovery system. The increased expense of refining metals contaminated feedstocks due to these three factors lays a heavy economic burden on the refiner. Thus it would be desirable to find a way to eliminate metals contamination of the feedstock or to modify the catalyst in such a way as to passivate the aforementioned undesirable effects of the metal contaminants.
One method disclosed in U.S. Pat. Nos. 3,162,595; 3,162,596 and 3,165,462 is to remove the metals from the catalyst after the catalyst exits the reactor for regeneration. This is accomplished by a so-called demetallization process involving such steps as acid-washing, chlorinating, etc. to convert the metals on the catalyst to dispersable or volatile forms and separating the dissolved or dispersed metal poisons from the catalyst. This technology has not been widely used, presumably because of the expense involved.
Another method is to passivate the metal contaminants, or more specifically to ameliorate the undesirable effects thereof, by adding a passivating agent to the fresh catalyst, to the feedstock directly to impregnate the catalyst, or to regenerated catalyst, or to used cracking catalyst fines which are then added to the process. These passivating agents are metal compounds exemplified by an antimony tris (0,0-dihydrocarbylphosphorodithioate) disclosed in the following U.S. patents to McKay et al: Nos. 4,207,204; 4,031,002 and 4,025,458. The use of antimony compounds on catalyst fines is disclosed in U.S. Pat. No. 4,216,120 to Nielsen et al, and antimony compounds useful in restoring activity of used cracking catalyst is disclosed in U.S. Pat. No. 3,711,422 to Johnson.
Other passivating agents have also found utility for cracking catalysts. Bismuth and manganese compounds are disclosed by Readal et al in U.S. Pat. No. 3,977,963, and by McKinney et al in U.S. Pat. No. 4,083,807; and exclusive use of low levels of boron compounds are disclosed in U.S. Pat. No. 4,192,770 to Singleton. Tin compounds are disclosed in U.S. Pat. No. 4,040,945 to McKinney, and tin in combination with antimony is disclosed in U.S. Pat. No. 4,255,287 to Bertus et al. A thallium supplying material is disclosed in U.S. Pat. No. 4,238,367 to Bertus et al for passivation of contaminant metals.
Treating non-zeolitic cracking catalysts with phosphorus compounds is also known. For example U.S. Pat. No. 2,758,097 to Doherty et al discloses addition of P.sub.2 O.sub.5 or compounds convertible to P.sub.2 O.sub.5 to reduce the undesirable effects of nickel on nickel-poisoned siliceous cracking catalysts. U.S. Pat. No. 2,977,322 to Varvel et al discloses a method for deactivating metal poisons by contacting a clay catalyst with phosphorus in combination with chlorine compounds. U.S. Pat. No. 2,921,018 to Helmers et al discloses treating acid-activated clay with compounds of phosphorus to convert metallic poisons to their corresponding phosphorus compounds, thereby deactivating the contaminant metals. These patents do not recognize that adding certain phosphorus compounds, particularly phosphoric acids, can destroy the zeolite in zeolitic cracking catalysts after heat treatment.
Other methods of incorporating phosphorus into or onto cracking catalyst have been tried. U.S. Pat. Nos. 4,158,621 and 4,228,036 both to Swift et al disclose a silica-alumina-aluminum phosphate matrix incorporating a zeolite having cracking activity. In U.S. Pat. Nos. 4,179,358 and 4,222,896 both to Swift et al a magnesia-alumina-aluminum phosphate matrix composited with a zeolite having cracking activity is disclosed.
In U.S. Pat. No. 3,867,279 to Young a zeolite cracking catalyst containing 1-30% P.sub.2 O.sub.5 for improved crush strength is disclosed. No utility of phosphorus for metals passivation is recognized in this patent.
It is an object of this invention to provide a method for controlling the detrimental effects of metallic contaminants, especially vanadium, on cracking catalysts, particularly zeolitic cracking catalysts.
Another object of the present invention is to provide a means by which phosphorus compounds may be incorporated into zeolitic cracking catalysts with minimized zeolite destruction.
Still another object of the present invention is to provide additional operational flexibility to catalytic cracking units limited by regenerator capacity by substitution of a portion of other known passivators by the phosphorus compounds of this invention.